Positive-electrode material for lithium ion secondary battery, method for producing the same, and lithium ion secondary battery

ABSTRACT

A positive-electrode material for a lithium ion secondary battery contains a lithium complex compound that is represented by the formula: Li 1+a Ni b Mn c Co d Ti e M f O 2+α , and has an atomic ratio Ti 3+ /Ti 4+  between Ti 3+  and Ti 4+ , as determined through X-ray photoelectron spectroscopy, of greater than or equal to 1.5 and less than or equal to 20. In the formula, M is at least one element selected from the group consisting of Mg, Al, Zr, Mo, and Nb, and a, b, c, d, e, f, and α are numbers satisfying −0.1≤a≤0.2, 0.7&lt;b≤0.9, 0≤c&lt;0.3, 0≤d&lt;0.3, 0&lt;e≤0.25, 0≤f&lt;0.3, b+c+d+e+f=1, and −0.2≤α≤0.2.

TECHNICAL FIELD

The present invention relates to a positive-electrode material used fora positive electrode of a lithium ion secondary battery, a method forproducing the positive-electrode material, and a lithium ion secondarybattery that uses the positive-electrode material.

BACKGROUND ART

Lithium ion secondary batteries are in widespread use as compact,lightweight secondary batteries with high energy density. For lithiumion secondary batteries, for example, lithium metal, lithium alloy,metal oxide, carbon, or the like is used as a negative-electrodematerial, while lithium-metal composite oxide with a layered structureis used as a positive-electrode material. As an example of such apositive-electrode material, there is disclosed a positive-electrodeactive material that exhibits high thermal stability when used for apositive electrode of a nonaqueous-electrolyte secondary battery, andhas a high charge-discharge capacity (see Patent Literature 1 below).

The positive-electrode active material for a nonaqueous-electrolytesecondary battery described in Patent Literature 1 contains a powder oflithium-metal composite oxide represented by the general formula:LiNi_(x)M_(1-x)O₂ (x in the formula satisfies (4−Z)×x≥0.75, where Z isthe average valence of Ni, and M in the formula represents at least oneelement whose average valence in the entire M is greater than or equalto 3). When a nonaqueous-electrolyte secondary battery that uses such apowder as a positive-electrode active material is charged up to acomposition of Li_(0.25)Ni_(x)M_(1-x)O₂, the number of moles ofquadrivalent Ni becomes less than or equal to 60% of the total number ofmoles of Ni and M.

According to Patent Literature 1, using the aforementionedpositive-electrode active material can prevent a decrease in the initialcapacity of the battery that would occur due to substitution of Ni withanother element, and can, when the material is used for a positiveelectrode of a lithium ion battery, improve the thermal stability of thebattery as long as the material satisfies the following conditions: thenumber of moles of quadrivalent Ni, which is thermally unstable, becomesless than or equal to 60% of the total number of moles of Ni and theadded element M when the battery is charged up to a composition ofLi_(0.25)Ni_(x)M_(1-x)O₂ that indicates the fully charged state.

There is also disclosed a method for producing surface-modifiedlithium-containing composite oxide that contains lithium-containingcomposite oxide particles and lithium-titanium composite oxiderepresented by the general formula: Li_(p)N_(x)M_(y)O_(z)F_(a) on thesurface layers of the particles (see Patent Literature 2 below). In thegeneral formula, N is at least one element selected from the groupconsisting of Co, Mn, and Ni, and M is at least one element selectedfrom the group consisting of a transition metal element other than Co,Mn, and Ni; Al; Sn; and an alkaline-earth metal element, and satisfies0.9≤p≤1.3, 0.9≤x≤2.0, 0≤y≤0.1, 1.9≤z≤4.2, and 0≤a≤0.05.

In the production method in accordance with the invention described inPatent Literature 2, a powder of lithium-containing composite oxide isfirst impregnated with a solution containing a lithium source and atitanium source dissolved therein. Then, heat treatment at 400 to 1000°C. is applied to the obtained lithium titanium-impregnated particles.The invention described in Patent Literature 2 is characterized in thatthe titanium content in the surface layer of the surface-modifiedlithium-containing composite oxide, which is obtained through the heattreatment, is 0.01 to 1.95 mol % relative to the lithium-containingcomposite oxide that is the base material. Accordingly, in PatentLiterature 2, a method for producing surface-modified lithium-containingcomposite oxide is provided that can be advantageously used for apositive electrode of a lithium ion secondary battery, has a highdischarge capacity and volume capacity density, is highly safe, and hasexcellent charge-discharge cycle durability, rate characteristics, andlow production cost.

CITATION LIST Patent Literature

Patent Literature 1: JP 2006-107818 A

Patent Literature 2: WO 2009/057722 A

SUMMARY OF INVENTION Technical Problem

According to the positive-electrode material described in PatentLiterature 1, deterioration of the lithium ion secondary battery alongwith the charge-discharge cycles can be suppressed by substituting Niwith another element. However, there is a problem in that as the amountof Ni that contributes to charge-discharge reactions decreases, it isdifficult to increase the capacity.

In the positive-electrode material described in Patent Literature 2, thesurfaces of the lithium-containing composite oxide particle are eachmodified with a surface layer containing lithium-titanium compositeoxide, so that the charge-discharge cycle durability is improved.However, as the surface layer having a different crystal structure fromthat of the positive-electrode material inhibits insertion anddesorption of lithium ions, there is a possibility that the resistanceduring charging and discharging may increase. In addition, there is alsoa possibility that if an excessive amount of titanium is added, theamount of Ni that contributes to charge-discharge reactions maydecrease, which in turn may decrease the charge-discharge capacity.Further, if the surface is excessively modified with lithium-titaniumcomposite oxide, the amount of lithium that can be inserted or desorbedmay decrease, which in turn may decrease the charge-discharge capacity.

The present invention has been made in view of the foregoing problems.It is an object of the present invention to provide a positive-electrodematerial for a lithium ion secondary battery that has a highercharge-discharge capacity than those of the conventional lithium ionsecondary batteries and also has a suppressed resistance increase rateand excellent cycle characteristics; a method for producing thepositive-electrode material; and a lithium ion secondary battery thathas excellent low-temperature output characteristics.

Solution to Problem

In order to achieve the aforementioned object, a positive-electrodematerial for a lithium ion secondary battery of the present inventioncontains a lithium complex compound represented by the following Formula(1) and having an atomic ratio Ti³⁺/Ti⁴⁺ between Ti³⁺ and Ti⁴⁺, asdetermined through X-ray photoelectron spectroscopy, of greater than orequal to 1.5 and less than or equal to 20.Li_(1+a)Ni_(b)Mn_(c)Co_(d)Ti_(e)M_(f)O_(2+α)  (1)

It should be noted that in Formula (1) above, M is at least one elementselected from the group consisting of Mg, Al, Zr, Mo, and Nb, and a, b,c, d, e, f, and a are numbers satisfying −0.1≤a≤0.2, 0.7<b≤0.9, 0≤c<0.3,0≤d<0.3, 0<e≤0.25, 0≤f<0.3, b+c+d+e+f=1, and −0.2≤α≤0.2.

Advantageous Effects of Invention

The present invention can provide a positive-electrode material for alithium ion secondary battery that has a higher charge-dischargecapacity than those of the conventional lithium ion secondary batteriesand also has a suppressed resistance increase rate and excellent cyclecharacteristics; a method for producing the positive-electrode material;and a lithium ion secondary battery that has excellent low-temperatureoutput characteristics.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a flowchart illustrating each step of a method for producinga positive-electrode material in accordance with an embodiment of thepresent invention.

FIG. 1B is a flowchart illustrating each step of a variation of theproduction method shown in FIG. 1A.

FIG. 2 is a schematic partial cross-sectional view of a lithium ionsecondary battery and a positive electrode in accordance with anembodiment of the present invention.

FIG. 3A is a photograph of a cross-section of secondary particlesobtained when titanium oxide is used in a mixing step.

FIG. 3B is a photograph of a cross-section of secondary particlesobtained when an organic titanium compound is used in a mixing step.

FIG. 4A is a mapped image of a Ti element in a primary particle of apositive-electrode material of Example 4.

FIG. 4B is a graph illustrating the distance from the surface of theprimary particle shown in FIG. 4A, and the composition ratio.

FIG. 5 is a graph showing the XRD spectrum of the positive-electrodematerial of Example 4.

FIG. 6 is a graph showing the particle fracture strength of each ofpositive-electrode materials of Examples and Comparative Examples.

FIG. 7 is a graph showing the relationship between the resistanceincrease rate of each secondary battery and the specific surface area ofeach positive-electrode material.

FIG. 8A is a photograph of a cross-section of an area around the surfaceof a particle of a positive-electrode material of Example 2 after 300cycles.

FIG. 8B is a photograph of a cross-section of an area around the surfaceof a particle of a positive-electrode material of Comparative Example 1after 300 cycles.

FIG. 9A is a photograph of a cross-section of the positive-electrodematerial of the secondary battery of Example 2 after 0 cycle.

FIG. 9B shows the measurement result of TEM-EELS at each distance fromthe surface of the positive-electrode material in FIG. 9A.

FIG. 9C shows the measurement result of TEM-EELS at each distance fromthe surface of the positive-electrode material in FIG. 9A.

FIG. 10A is a photograph of a cross-section of the positive-electrodematerial for the secondary battery of Comparative Example 1 after 0cycle.

FIG. 10B shows the measurement result of TEM-EELS at each distance fromthe surface of the positive-electrode material in FIG. 10A.

FIG. 10C shows the measurement result of TEM-EELS at each distance fromthe surface of the positive-electrode material in FIG. 10A.

FIG. 11A is a photograph of a cross-section of the positive-electrodematerial of the secondary battery of Example 2 after 300 cycles.

FIG. 11B shows the measurement result of TEM-EELS at each distance fromthe surface of the positive-electrode material in FIG. 11A.

FIG. 11C shows the measurement result of TEM-EELS at each distance fromthe surface of the positive-electrode material in FIG. 11A.

FIG. 12A is a photograph of a cross-section of the positive-electrodematerial of the secondary battery of Comparative Example 1 after 300cycles.

FIG. 12B shows the measurement result of TEM-EELS at each distance fromthe surface of the positive-electrode material in FIG. 12A.

FIG. 12C shows the measurement result of TEM-EELS at each distance fromthe surface of the positive-electrode material in FIG. 12A.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of a positive-electrode material for a lithiumsecondary battery and a method for producing the positive-electrodematerial of the present invention will be described in detail.

(Positive-Electrode Material for Lithium Secondary Battery)

A positive-electrode material in this embodiment is, for example, apowder-form positive-electrode active material used for a positiveelectrode of a lithium ion secondary battery described below. Thepositive-electrode material in this embodiment contains a lithiumcomplex compound represented by the following Formula (1) and having anatomic ratio (Ti³⁺/Ti⁴⁺) between trivalent titanium Ti (Ti³⁺) andquadrivalent titanium (Ti⁴⁺), as determined on the basis of X-rayphotoelectron spectroscopy (XPS), of greater than or equal to 1.5 andless than or equal to 20.Li_(1+a)Ni_(b)Mn_(c)Co_(d)Ti_(e)M_(f)O_(2+α)  (1)

It should be noted that in Formula (1) above, M is at least one elementselected from the group consisting of Mg, Al, Zr, Mo, and Nb, where a,b, c, d, e, f, and a are numbers satisfying −0.1≤a≤0.2, 0.7<b≤0.9,0≤c<0.3, 0≤d<0.3, 0<e≤0.25, 0≤f<0.3, b+c+d+e+f=1, and −0.2≤α≤0.2.Hereinafter, the defined range of a, b, c, d, e, f and a will bedescribed in detail.

In Formula (1) above, a indicates the stoichiometric ratio of thepositive-electrode material represented by the chemical formula: LiM′O₂,that is, the amount of excess or deficiency of Li compared toLi:M′:O=1:1:2. Herein, M′ indicates a metal element other than Li inFormula (1). As the Li content is higher, the valence of the transitionmetal before the battery is charged becomes higher. Thus, the rate ofchange in the valence of the transition metal upon desorption of Li isreduced, and the charge-discharge cycle characteristics of thepositive-electrode material can thus be improved. To the contrary, asthe Li content is higher, the charge-discharge capacity of thepositive-electrode material will be decrease. Meanwhile, if the Licontent is significantly lower than the stoichiometric ratio, thecharge-discharge capacity will decrease with a decrease in the Licontent. Thus, setting a, which represents the amount of excess ordeficiency of Li in Formula (1) above, in the range of −0.1 to 0.2 canimprove the charge-discharge cycle characteristics of the material andsuppress a reduction in the charge-discharge capacity.

More preferably, a, which represents the amount of excess or deficiencyof Li in Formula (1) above, can be set in the range of −0.05 to 0.1, ormore preferably, in the range of 0 to 0.06. When a in Formula (1) aboveis greater than or equal to −0.05, or more preferably, greater than orequal to 0 and less than or equal to 0.06, a sufficient amount of Lithat can contribute to charging and discharging is secured, and thecapacity of the positive-electrode material can thus be increased. Inaddition, when a in Formula (1) above is less than or equal to 0.1,charge compensation along with a change in the valence of the transitionmetal can be sufficiently secured, and thus, both a high capacity andhigh charge-discharge cycle characteristics can be achieved.

When b, which indicates the Ni content in Formula (1) above, is over0.7, a sufficient amount of Ni that can contribute to charging anddischarging can be secured in the positive-electrode material, which isadvantageous for increasing the capacity. Meanwhile, if b in Formula (1)above is over 0.9, there is a possibility that Ni may be partiallysubstituted with a Li site, and thus, a sufficient amount of Li that cancontribute to charging and discharging may not be secured, and thecharge-discharge capacity of the positive-electrode material maydecrease. Therefore, setting b, which indicates the Ni content inFormula (1) above, to greater than 0.7 and less than or equal to 0.9, orpreferably in the range of 0.75 to 0.85 can increase the capacity of thepositive-electrode material and suppress a decrease in thecharge-discharge capacity.

In addition, adding Mn has a function of stably maintaining the layeredstructure even when desorption of Li occurs due to charging. However, ifc, which indicates the Mn content in Formula (1) above, is greater thanor equal to 0.3, the capacity of the positive-electrode material willdecrease. Therefore, setting c in Formula (1) above to greater than orequal to 0 and less than 0.3 can stably maintain the layered structureof the lithium complex compound that forms the positive-electrodematerial even when insertion or desorption of Li occurs due to chargingor discharging, and thus can suppress a decrease in the capacity of thepositive-electrode material.

The range of d, which indicates the Co content in Formula (1) above, ispreferably greater than or equal to 0 and less than 0.3. If d is greaterthan or equal to 0.3, the supply amount will be limited, and theproportion of Co, which is costly, will be relatively high, which isdisadvantageous in the industrial production of the positive-electrodematerial.

The range of e, which indicates the Ti content in Formula (1) above, isgreater than 0 and less than or equal to 0.25, or more preferably,greater than or equal to 0.001 and less than or equal to 0.15. From aperspective of surely obtaining the advantageous effect of adding Ti, eis further preferably greater than or equal to 0.005 and less than orequal to 0.15. From a perspective of suppressing an increase in thematerial cost and improving the sintering property of thepositive-electrode material, e is further preferably greater than orequal to 0.001 and less than or equal to 0.05.

Although Li and Ti can form a variety of Li—Ti—O compounds, Ti is moststable when it is quadrivalent (Ti⁴⁺) and is likely to form a compoundsuch as Li₂TiO₃ or Li₄Ti₅O₁₂. If e is over 0.25, such Li—Ti—O compoundis likely to be generated as a different phase, and the resistance thusbecomes high. Further, as the cost is increased, such a compound is notpreferable as an industrial material.

In addition, the ratio a/e between a, which indicates the amount ofexcess of Li, and e, which indicates the Ti content, in Formula (1)above is preferably less than or equal to 5 (a/e≤5). When the value ofa/e is less than or equal to 5, generation of a different phase like aLi—Ti—O compound during the stage of synthesizing a lithium complexcompound can be suppressed, and the strength of the particles can thusbe improved. Consequently, a decrease in the capacity and an increase inthe resistance along with the charge-discharge cycles can be suppressed.

Further, when M in Formula (1) above is at least one metal elementselected from the group consisting of Mg, Al, Zr, Mo, and Nb,electrochemical activity of the positive-electrode material can besecured. In addition, when a metal site of the lithium complex compoundthat forms the positive-electrode material is substituted with suchmetal element, the stability of the crystal structure of the lithiumcomplex compound as well as the electrochemical characteristics (e.g.,cycle characteristics) of the layered positive-electrode active materialcan be improved. It should be noted that if f, which indicates the Mcontent in Formula (1) above, is excessive, the capacity of thepositive-electrode material will decrease. Therefore, when b, c, d, e,and f in Formula (1) above satisfy b+c+d+e+f=1, a decrease in thecapacity of the positive-electrode material can be suppressed.

a in Formula (1) above indicates the range that permits alayer-structured compound included in the space group R-3 m, andindicates the amount of excess or deficiency of oxygen. From aperspective of maintaining an α-NaFeO₂-type layered structure of thelithium complex compound that forms the positive-electrode material, amay be in the range of −0.2 to 0.2, for example. However, when a inFormula (1) above is in the range of −0.1 to 0.1, the layered structureof the lithium complex compound that forms the positive-electrodematerial can be maintained more surely.

It should be noted that the particles of the lithium complex compoundthat forms the positive-electrode material, which is the powder-formpositive-electrode active material, may be primary particles in whichindividual particles are separate from one another. However, theparticles are desirably secondary particles obtained by binding aplurality of primary particles together through sintering or the like.The primary particles or secondary particles may also contain anunavoidable free lithium compound.

The average particle size of the primary particles of thepositive-electrode material is preferably greater than or equal to 0.1μm and less than or equal to 2 μm, for example. When the averageparticle size of the primary particles of the positive-electrodematerial is less than or equal to 2 μm, a reaction site of thepositive-electrode material can be secured, and a high capacity and lowresistance are obtained. In addition, the average particle size of thesecondary particles of the positive-electrode material is preferablygreater than or equal to 3 μm and less than or equal to 50 μm, forexample.

The particles of the positive-electrode material can be formed assecondary particles by granulating primary particles, which have beenproduced with a positive-electrode material production method describedbelow, through dry granulation or wet granulation. As a means forgranulation, granulators such as spray dryers or fluid bed granulatorscan be used, for example.

The crystal structures of the particles of the positive-electrodematerial can be confirmed using X-ray diffraction (XRD), for example. Inaddition, the average composition of the particles of thepositive-electrode material can be confirmed using high-frequencyinductively coupled plasma (ICP), atomic absorption spectrometry (AAS),or the like. The average particle size of the particles of thepositive-electrode material can be measured using a laser diffractionparticle size distribution measuring apparatus, for example.

The BET specific surface area of the positive-electrode material ispreferably about greater than or equal to 0.2 m²/g and less than orequal to 2.0 m²/g, for example. When the BET specific surface area ofthe particles of the positive-electrode material is less than or equalto about 2.0 m²/g, the filling property of the positive-electrodematerial of the positive electrode can be improved, and thus, a positiveelectrode with high energy density can be produced. Further, as the areain contact with an electrolytic solution does not become excessive, sidereactions with the electrolytic solution can be suppressed, and anincrease in the resistance can thus be suppressed. More preferably, theBET specific surface area of the positive-electrode material is greaterthan or equal to 0.5 m²/g and less than or equal to 1.5 m²/g. It shouldbe noted that the BET specific surface area can be measured using anautomatic specific surface area measuring apparatus.

Further, the particle fracture strength of the positive-electrodematerial is preferably greater than or equal to 50 MPa and less than orequal to 200 MPa. Accordingly, the particles of the positive-electrodematerial will not become damaged during the process of producing anelectrode, and thus, coating failures, such as peeling, can besuppressed in forming a positive-electrode mixture layer by coating thesurface of a positive-electrode current collector with a slurrycontaining the positive-electrode material. Further, since cracking ofthe secondary particles due to expansion or contraction of thepositive-electrode material along with charging and discharging can besuppressed, a decrease in the capacity and an increase in the resistancealong with the cycles can be suppressed. The particle fracture strengthof the positive-electrode material can be measured using a microcompression testing machine, for example.

In addition, the positive-electrode material preferably has aconcentrated Ti³⁺ layer on the surface of each secondary particle thatis the agglomerated primary particles of the lithium complex compound.It is effective if the concentrated Ti³⁺ layer is provided on a surface,which is in contact with an electrolytic solution, of each secondaryparticle formed by the agglomerated primary particles, and further, theconcentrated Ti³⁺ layer may also be provided inside the secondaryparticle. The concentrated Ti³⁺ layer is preferably in the form in whicha transition metal site in the layered structure of the lithium complexcompound included in the space group R-3m is substituted with Ti.Conversely, if the concentrated Ti³⁺ layer has a structure other thanthe R-3m layered structure, the layer becomes a different phase, whichis undesirable as the discharge capacity will decrease. In addition, thesurfaces of the secondary particles of the positive-electrode materialmay be fluorinated.

Hereinafter, the function of the positive-electrode material in thisembodiment will be described.

The positive-electrode material in this embodiment contains Ni in therange in which b, which indicates the Ni content in the lithium complexcompound represented by Formula (1) above, is greater than 0.7 and lessthan or equal to 0.9. Ni is contained mainly as trivalent Ni (Ni³⁺) inthe lithium complex compound. Ni³⁺ in the lithium complex compoundbecomes quadrivalent Ni (Ni⁴⁺) from Ni³⁺ when the lithium ion secondarybattery is charged, and undergoes an oxidation-reduction reaction inwhich Ni returns to Ni³⁺ from Ni⁴⁺ when the lithium ion secondarybattery is discharged, and thus contributes to increasing thecharge-discharge capacity of the positive-electrode material. However,Ni³⁺ in the lithium complex compound is likely to become divalent Ni(Ni²⁺), which is stable, when the lithium ion secondary battery isrepeatedly charged and discharged over a certain number of times, andthus generates NiO-like cubic crystals by releasing oxygen from thecrystals. Ni²⁺ whose valence has changed with a change in the crystalstructure does not contribute to charging or discharging of the lithiumion secondary battery any more.

Typically, a positive-electrode material represented by the generalformula: LiNiO₂ has a problem in that although it has a higher capacitythan a positive-electrode material represented by the general formula:LiCoO₂, it has lower stability in the crystal structure than that of thepositive-electrode material represented by LiCoO₂, and thus undergoessignificant deterioration along with the charge-discharge cycles. Thisis because, in the positive-electrode material represented by LiNiO₂,some of Ni³⁺ in the transition metal site will easily move as Ni²⁺ tothe Li site (cation mixing) as described above, and release oxygen atlower temperatures in the charged state in comparison with thepositive-electrode material represented by LiCoO₂. In addition, thepositive-electrode material represented by LiNiO₂ has a possibility thata film of a decomposed matter of the electrolytic solution may be formedon the positive-electrode material due to a reaction between theelectrolytic solution and oxygen released from the surfaces of theparticles of the positive-electrode material along with thecharge-discharge cycles, or a NiO-like different phase may be formed onthe surface of the positive-electrode material, which may inhibit themovement of electric charges.

In order to solve the aforementioned problems, the positive-electrodematerial in this embodiment contains Ti in the range in which e, whichindicates the Ti content in the lithium complex compound represented byFormula (1) above, is greater than 0 and less than or equal to 0.25. Tiin the lithium complex compound that forms the positive-electrodematerial is contained mainly in the state of Ti³⁺ or Ti⁴⁺. Ti³⁺ in thelithium complex compound can, by becoming more stable Ti⁴⁺ and thuscarrying out charge compensation, cause a change in the valence fromNi³⁺ to Ni²⁺ while maintaining the layered structure of the lithiumcomplex compound, and thus suppress generation of a NiO-like differentphase, thereby contributing to suppressing a decrease in the capacityand suppressing an increase in the resistance along with thecharge-discharge cycles. That is, as Ti³⁺ is present in large quantitieson the surface layer of the positive-electrode material, theaforementioned charge compensation is effectively carried out, and thelayered structure is thus stabilized. In addition, generation of aNiO-like different phase on the surface of the positive-electrodematerial can be suppressed.

The range of e, which indicates the Ti content, is preferably greaterthan 0 and less than or equal to 0.25, or more preferably, greater thanor equal to 0.005 and less than or equal to 0.15. Further preferably,the range of e, which indicates the Ti content, is greater than or equalto 0.005 and less than or equal to 0.05. Such a range can obtainappropriate electrode characteristics without the synthesis conditionsgreatly changed.

However, as described above, Ti is most stable when it is quadrivalent(Ti⁴⁺) and is likely to form a compound such as Li₂TiO₃ or Li₄Ti₅O₁₂. Itwas found that a different phase like Li₄Ti₅O₁₂ herein is generated inthe grain boundaries between the primary particles, and this causes adecrease in the strength of the secondary particles. That is, if e isover 0.25, a different phase like Li₂TiO₃ is likely to be generated, andconsequently, the resistance becomes high.

Meanwhile, it was also found that if the Li content is excessive, theproportion of a different phase like Li₂TiO₃ that remains in the grainboundaries becomes high, and this is related to the balance between theLi content and the Ti content. That is, the ratio a/e between a, whichindicates the amount of excess of Li, and e, which indicates the Ticontent, in Formula (1) above is preferably less than or equal to 5(a/e≤5). If the value of a/e is over 5, a different phase like a Li—Ti—Ocompound is likely to be generated during the stage of synthesizing alithium complex compound, and the strength of the particles will thusdecrease. Consequently, a decrease in the capacity and an increase inthe resistance are likely to occur along with the cycles.

Further, the positive-electrode material in this embodiment contains alithium complex compound that is represented by Formula (1) above andhas an atomic ratio Ti³⁺/Ti⁴⁺ between Ti³⁺ and Ti⁴⁺, as determined onthe basis of X-ray photoelectron spectroscopy (XPS) that is a method ofanalyzing a surface state, of greater than or equal to 1.5 and less thanor equal to 20. The reason why Ti³⁺ is contained in large quantities,specifically, as much as 1.5 to 20 times that of Ti⁴⁺, which is moststable as Ti, is estimated that Ti has solid-dissolved in thepositive-electrode material and Ti³⁺ has thus been stabilized.

Accordingly, when Ni²⁺ is generated in the lithium complex compoundalong with the charge-discharge cycles of the positive-electrodematerial, the layered structure of the lithium complex compound can bemaintained as long as Ti³⁺ becomes Ti⁴⁺ and charge compensation is thuscarried out. Further, as release of oxygen along with a change in thestructure of the positive-electrode material can be suppressed,decomposition reactions of the electrolytic solution along with thecharge-discharge cycles can be suppressed. Thus, according to thepositive-electrode material in this embodiment, excellent cyclecharacteristics can be exhibited.

It should be noted that if the atomic ratio Ti³⁺/Ti⁴⁺ between Ti³⁺ andTi⁴⁺ in the lithium complex compound is less than 1.5, the effect ofsuppressing a decrease in the charge-discharge capacity of thepositive-electrode material, which would occur as Ni³⁺ in the lithiumcomplex compound becomes Ni²⁺, cannot be insufficiently obtained, andthus, it is difficult to obtain a higher charge-discharge capacity thanthose of the conventional lithium ion secondary batteries. In addition,as a phase with a different crystal structure from that of thepositive-electrode material is generated on the surface of thepositive-electrode material, the initial resistance will increase, orthe bonding strength between the primary particles will weaken, which inturn will decrease the strength of the particles. Meanwhile, if theatomic ratio Ti³⁺/Ti⁴⁺ between Ti³⁺ and Ti⁴⁺ in the lithium complexcompound is over 20, there is a possibility that the charge-dischargecapacity of the positive-electrode material may decrease along with anexcessive growth of the sintered particles in the lithium complexcompound.

Further, as the surface of each secondary particle of thepositive-electrode material has a concentrated Ti³⁺ layer, thecharge-discharge cycle characteristics can be improved. In addition, asthe concentrated Ti layer on the surface of each secondary particle ofthe positive-electrode material can stabilize the layered structure andthus does not inhibit insertion or desorption lithium ions, an increasein the resistance during charging and discharging can be suppressed.

In addition, as the surface (a further front surface of the concentratedTi³⁺ layer) of each secondary particle of the positive-electrodematerial is fluorinated, the surface of the particle of thepositive-electrode material is modified, and elution of transition metalas well as decomposition of the nonaqueous solvent is suppressed.Accordingly, the cycle characteristics of the secondary battery areimproved.

(Method for Producing Positive-Electrode Material for Lithium SecondaryBattery)

FIG. 1A is a flowchart illustrating each step of a method for producinga positive-electrode material for a lithium secondary battery in thisembodiment. The method for producing a positive-electrode material inthis embodiment is a method for producing a positive-electrode materialthat is the aforementioned powder-form positive-electrode activematerial, and mainly includes a mixing step S1 and a firing step S2. Inaddition, as shown in FIG. 1B, the method for producing apositive-electrode material in this embodiment may also include animmersing step S3 in addition to the mixing step S1 and the firing stepS2.

In the mixing step S1, a mixture is obtained by mixing alithium-containing compound with compounds each containing a metalelement other than Li in Formula (1) above. As the lithium-containingcompound, lithium carbonate can be used, for example. Lithium carbonateas a starting material of the positive-electrode material is excellentin industrial applicability and practical utility in comparison withother Li-containing compounds, such as acetic acid lithium, nitric acidlithium, lithium hydroxide, lithium chloride, and sulfuric acid lithium.

As the compounds each containing a metal element other than Li inFormula (1) above, a Ni-containing compound, a Mn-containing compound, aCo-containing compound, a Ti-containing compound, a M-containingcompound, and the like can be used, for example. Herein, theM-containing compound is a compound containing at least one metalelement selected from the group consisting of Mg, Al, Zr, Mo, and Nb.

As each of the Ni-containing compound, the Mn-containing compound, andthe Co-containing compound, oxide, hydroxide, carbonate, sulfate, oracetate can be used, for example. In particular, oxide, hydroxide, orcarbonate is preferably used. Meanwhile, as the M-containing compound,acetate, nitrate, carbonate, sulfate, oxide, or hydroxide can be used,for example. In particular, carbonate, oxide, or hydroxide is preferablyused.

Further, the Ti-containing compound can be at least one compoundselected from the group consisting of oxide, nitride, carbide, and anorganic titanium compound, for example, and is preferably Ti oxide or anorganic titanium compound, for example. Examples of the organic titaniumcompound include a Ti-containing coupling agent, a Ti-containingalkoxide, a Ti-containing chelating agent, a Ti-containing acylatingagent, and a Ti-containing surfactant. The organic titanium compound canbe mixed in a liquid-state in the mixing step S1.

In the mixing step S1, a raw material powder is prepared by mixing theaforementioned starting materials weighed at a ratio to obtain apredetermined composition corresponding to Formula (1) above. In themixing step S1, the aforementioned starting materials are preferablymixed by being ground with a grinder, for example. Accordingly, auniformly mixed power-form solid mixture can be prepared. As a grinderfor grinding the compound of the aforementioned starting materials, acommon precision grinder, such as a ball mill, a jet mill, or a sandmill, can be used.

The starting materials are preferably ground through wet grinding. Froman industrial perspective, a solvent used for wet grinding is preferablywater. A solid-liquid mixture obtained by grinding the aforementionedstarting materials through wet grinding can be dried with a dryer, forexample. As the dryer, a spray dryer, a fluidized-bed dryer, or anevaporator can be used, for example.

When a solid material such as oxide is used as the Ti-containingcompound in the mixing step S1, dispersibility is likely to be lowerthan that when a liquid material is used. However, dispersibility can beimproved by adjusting the grinding conditions.

In the firing step S2, the mixture obtained in the mixing step S1 isfired under an oxidizing atmosphere to obtain a lithium complex compoundthat is represented by Formula (1) above and has an atomic ratioTi³⁺/Ti⁴⁺ between Ti³⁺ and Ti⁴⁺, as determined on the basis of XPS, ofgreater than or equal to 1.5 and less than or equal to 20. The oxygenconcentration in the oxidizing atmosphere in the firing step S2 ispreferably greater than or equal to 80% from a perspective ofsufficiently promoting a Ni oxidation reaction, and the oxygenconcentration is more preferably greater than or equal to 90%, orfurther preferably greater than or equal to 95%, or even more preferably100%.

The heat treatment temperature (firing temperature; hereinafter, thesame) in the firing step S2 is preferably greater than or equal to 700°C. and less than 900° C. If the heat treatment temperature is less than700° C., the lithium complex compound cannot be crystallizedinsufficiently. If the heat treatment temperature is over 900° C., thelayered structure of the lithium complex compound will be decomposed andNi²⁺ will be generated. Thus, the capacity of the obtainedpositive-electrode material becomes low.

The appropriate value of the heat treatment temperature in the firingstep S2 differs depending on the amount of an unreacted Li raw material,and is influenced by the rate of temperature increase and the like. Ifthe amount of an unreacted Li raw material is large, dissolution of theLi raw material will occur, and the particles will be likely to grow dueto liquid-phase sintering. Excessive growth of the particles will leadto a decrease in the charge-discharge capacity. Therefore, anappropriate value of the heat treatment temperature will decrease.However, if the heat treatment temperature is low, Ti, which has beenadded, cannot be efficiently converted into Ti³⁺ in the lithium complexcompound, and thus, the charge-discharge capacity and the cyclecharacteristics of the positive-electrode material will decrease. Thus,the heat treatment temperature in the firing step S2 is preferablygreater than or equal to 750° C. and less than or equal to 850° C., forexample.

The firing step S2 may include a first heat treatment step S21, a secondheat treatment step S22, and a third heat treatment step S23. In thefirst heat treatment step S21, the mixture obtained in the mixing stepS1 is subjected to heat treatment at a heat treatment temperature ofgreater than or equal to 200° C. and less than or equal to 400° C. for aperiod of greater than or equal to 0.5 hour and less than or equal to 5hours, for example, whereby a first precursor is obtained. In the secondheat treatment step S22, the first precursor obtained in the first heattreatment step S21 is subjected to heat treatment at a heat treatmenttemperature of greater than or equal to 450° C. and less than or equalto 720° C. for a period of greater than or equal to 0.5 hour and lessthan or equal to 50 hours, whereby a second precursor is obtained.Through such heat treatment, the amount of an unreacted Li raw materialcan be controlled. In the third heat treatment step S23, the secondprecursor obtained in the second heat treatment step S22 is subjected toheat treatment at a heat treatment temperature of greater than or equalto 700° C. and less than or equal to 900° C. for a period of greaterthan or equal to 0.5 hour and less than or equal to 50 hours, whereby alithium complex compound is obtained. The heat treatment temperature inthe third heat treatment step S23 is preferably greater than or equal to750° C. from a perspective of efficiently converting Ti, which has beenadded, into Ti³⁺ in the lithium complex compound.

According to the method for producing a positive-electrode material inthis embodiment, a lithium complex compound that is represented byFormula (1) above and contains an atomic ratio Ti³⁺/Ti⁴⁺ between Ti³⁺and Ti⁴⁺, as determined on the basis of XPS, of greater than or equal to1.5 and less than or equal to 20 can be obtained by firing a mixture,which has been obtained by mixing predetermined starting materials at apredetermined ratio in the mixing step S1, under an oxidizing atmospherein the firing step S2. With the thus obtained lithium complex compound,a positive-electrode material, which is a powder-form positive-electrodeactive material, can be formed.

In particular, when an organic titanium compound is used as theTi-containing compound in the mixing step S1, the organic titaniumcompound can be mixed with the powder of the other starting materialsmore uniformly. More specifically, the organic titanium compound can bemixed in a liquid state in the mixing step S1, and thus can be dispersedmore uniformly in the mixture as compared to when other Ti-containingcompounds are used. Accordingly, a uniform solid-phase reaction of theTi-containing compound can be realized in the firing step S2, and thus,a lithium complex compound having an atomic ratio Ti³⁺/Ti⁴⁺ between Ti³⁺and Ti⁴⁺ of greater than or equal to 1.5 and less than or equal to 20can be obtained more efficiently.

Meanwhile, when titanium oxide is used as the Ti-containing compound,the amount of the component dissolved in the final drying in the mixingstep S1 becomes smaller than that when an organic titanium compound isused. Further, the amount of gas generated in the firing step S2 becomessmaller. Accordingly, voids become less likely to be generated in thesecondary particles, and the strength of the particles thus becomeslikely to be high.

FIG. 3A is a microscope photograph of the secondary particles obtainedwhen titanium oxide is used as the Ti-containing compound in the mixingstep S1. FIG. 3B is a microscope photograph of the secondary particlesobtained when an organic titanium compound is used as the Ti-containingcompound in the mixing step S1. Referring to the secondary particlesobtained when titanium oxide is used as the Ti-containing compound inmixing step S1 (FIG. 3A), generation of voids is suppressed as comparedto the secondary particles obtained when an organic titanium compound isused (FIG. 3B).

In addition, as shown in FIG. 1B, the method for producing apositive-electrode material in this embodiment may further include animmersing step S3. In the immersing step S3, the positive-electrodematerial for a lithium ion secondary battery, which has been producedthrough the mixing step S1 and the firing step S2, is immersed in anorganic solvent that contains dissolved therein a boroxine compoundrepresented by Formula (2) below and fluoride, and is then filtrated anddried, so that the surface of the positive-electrode material for thelithium ion secondary battery is treated.(BO)₃(OR)₃  (2)

It should be noted that R in Formula (2) above is an organic grouphaving 1 or more carbon atoms. Examples of the organic group (R) in theboroxine compound represented by Formula (2) above include astraight-chain or branched-chain alkyl group and a cycloalkylaryl group.Specific examples of such an organic group (R) include an ethyl group,n-propyl group, isopropyl group, n-butyl group, sec-butyl group,isobutyl group, and cyclohexyl group. The organic group (R) may alsoinclude halogen atoms typified by fluorine atoms, chlorine atoms, orbromine atoms; nitrogen atoms; and sulfur atoms.

The alkyl group may have a branched chain, and, if it has a branchedchain, a chain alkyl group of a portion that constitutes the straightchain has 3 or more carbon atoms. Although the upper limit of the numberof carbon atoms in the organic group that constitutes R is notparticularly limited, the upper limit (the upper limit of the totalnumber of carbon atoms in the organic group that constitutes R) ispreferably less than or equal to 6 from a perspective of facilitatingthe production. R may also have any substituent (halogen, nitrogen,sulfur, or the like).

Specific examples of the alkyl group include, but are not limited to, astraight-chain alkyl group represented by R=C_(a)H_(b) (where C is acarbon atom, H is a hydrogen atom, a is an integer of greater than orequal to 3, and b is a number satisfying b=2a+1), specifically, astraight-chain saturated hydrocarbon group or a branched-chain alkylgroup, such as a propyl group, butyl group, pentyl group, hexyl group,heptyl group, octyl group, nonyl group, or decyl group, and morespecifically, an isopropyl group, 1-methyl-propyl group, 1-ethyl-propylgroup, 2-methyl-propyl group, 1-methyl-butyl group, 1-ethyl-butyl group,2-methyl-butyl group, 2-ethyl-butyl group, 3-methyl-butyl group,1-methyl-pentyl group, 1-ethyl-pentyl group, 1-propyl-pentyl group,2-methyl-pentyl group, 2-ethyl-pentyl group, 2-propyl-pentyl group,3-methyl-pentyl group, 3-ethyl-pentyl group, 4-methyl-pentyl group,1-methyl-hexyl group, 1-ethyl-hexyl group, 1-propyl-hexyl group,1-butyl-hexyl group, 1-pentyl-hexyl group, 2-methyl-hexyl group,2-ethyl-hexyl group, 2-propyl-hexyl group, 2-butyl-hexyl group,3-methyl-hexyl group, 3-ethyl-hexyl group, 3-propyl-hexyl group,4-methyl-hexyl group, 4-ethyl-hexyl group, or 5-methyl-hexyl group.

As the boroxine compound, a compound having as an organic group (R) asecondary alkyl group with 1 to 6 carbon atoms is preferably used. Ifthe organic group (R) is primary, the molecular structure of theboroxine compound is unstable, and thus tends to be difficult to use.Meanwhile, if the organic group (R) is tertiary, the insolubility of theboroxine compound is high. Thus, the boroxine compound is difficult todissolve in an electrolytic solution. In contrast, if the organic group(R) is secondary, the boroxine compound is difficult to decompose andcan obtain appropriate solubility, which is advantageous. As theboroxine compound, tri-iso-propoxy boroxine (TiPBx) is preferably used.Among the boroxine compounds, a hydrocarbon group in which R has 2 to 6carbon atoms is preferably used.

Specific examples of the boroxine compound include trimethoxyboroxin((O—CH₃)₃(BO)₃), triethoxyboroxin ((O—CH₂CH₃)₃(BO)₃),triisopropoxyboroxin ((O—CH(CH₃)₂)₃(BO)₃), andtris(cyclohexyloxy)boroxine ((O—C₆H₁₁)₃(BO)₃).

The organic solvent may be any solvent that can maintain the solubilityof TiPBx. Examples of such solvent include an aprotic solvent. Forexample, dimethyl carbonate, acetone, acetonitrile, chloroform, ether,NMP, or dimethyl sulfoxide (DMSO) can be used.

Fluoride that is dissolved in the organic solvent together with theboroxine compound, such as TiPBx, is preferably lithiumhexafluorophosphate (LiPF₆), though not particularly limited thereto.TiPBx and LiPF₆ can be mixed at a molar ratio of 1:1, for example. Theimmersion time can be appropriately selected, for example, 30 minutes to6 hours, in accordance with the specific surface area of thepositive-electrode material or the concentration of the boroxinecompound. In drying after filtration, it is acceptable as long as theorganic solvent components can be removed. For example, drying isperformed in a vacuum at 120° C. for 1 hour with the temperature, time,and pressure appropriately selected.

When the surfaces of the particles of the positive-electrode materialare treated using a solvent containing TiPBx and LiPF₆, the surfaces ofthe particles are fluorinated, and a positive-electrode active materialhaving a boron-containing compound on the surfaces of the particles canbe obtained. Accordingly, the surfaces of the particles of thepositive-electrode material are modified, and elution of transitionmetal as well as decomposition of the nonaqueous solvent is suppressed.Accordingly, the cycle characteristics of the secondary battery areimproved.

Fluorination of the surfaces of the particles can be confirmed throughX-ray photoelectron spectroscopy (XPS) analysis. Specifically, whetherthe surfaces of the secondary particles have been fluorinated or not canbe confirmed with the following method.

Focusing on Ni that is the main component, a binding spectrum ofNi-2p2/3 is acquired. The spectrum is analyzed as superposed spectra ofthe following three components. A first component is a spectrum with abinding energy of 855.7±0.5 eV derived from Ni—O, a second component isa spectrum with a binding energy of 857.4±0.5 eV derived from Ni—F, anda third component is a spectrum with a binding energy of 862±0.5 eV thatis the average of the satellite peaks of the two components. Performingfitting analysis on the three superposed spectra and determining thearea ratio of the Ni—F spectrum using the total sum of the areas of thefirst and second spectra can determine the presence or absence offluorination.

(Positive Electrode and Lithium Ion Secondary Battery)

Hereinafter, a positive electrode for a lithium ion secondary batterythat uses the aforementioned positive-electrode material, and a lithiumion secondary battery that uses the positive electrode will bedescribed. FIG. 2 is a schematic partial cross-sectional view of thelithium ion secondary battery in this embodiment.

A lithium ion secondary battery 100 in this embodiment is cylindrical inshape, for example, and includes a cylindrical battery can 101 with abottom, which houses a nonaqueous electrolytic solution therein, a woundelectrode group 110 housed in the battery can 101, and a disk-shapedbattery lid 102 that seals an opening at the top of the battery can 101.The battery can 101 and the battery lid 102 are produced using ametallic material, such as stainless steel or aluminum, for example, andthe battery lid 102 is secured to the battery can 101 through swaging orthe like via a sealing material 106 made of an insulating resinmaterial, whereby the battery can 101 is sealed by and is electricallyinsulated from the battery lid 102. It should be noted that the shape ofthe lithium ion secondary battery 100 is not limited to a cylindricalshape, and any shape such as a rectangle, button, or laminate sheet canbe adopted.

The wound electrode group 110 is produced by winding a long-strip-shapedpositive electrode 111 and a long-strip-shaped negative electrode 112,which are arranged opposite each other with a long-strip-shapedseparator 113 interposed therebetween, around the central axis of thewinding. In the wound electrode group 110, a positive-electrode currentcollector 111 a is electrically connected to the battery lid 102 via apositive-electrode lead strip 103, and a negative-electrode currentcollector 112 a is electrically connected to the bottom of the batterycan 101 via a negative-electrode lead strip 104. Insulating plates 105for preventing short are arranged between the wound electrode group 110and the battery lid 102 and between the wound electrode group 110 andthe bottom of the battery can 101. The positive-electrode lead strip 103and the negative-electrode lead strip 104 are current extracting membersproduced using similar materials to those of the positive-electrodecurrent collector 111 a and the negative-electrode current collector 112a, respectively, and are bonded to the positive-electrode currentcollector 111 a and the negative-electrode current collector 112 a,respectively, through spot welding, ultrasonic welding, or the like.

The positive electrode 111 in this embodiment includes thepositive-electrode current collector 111 a and a positive electrodemixture layer 111 b formed on the surface of the positive-electrodecurrent collector 111 a. For the positive-electrode current collector111 a, a metal foil of aluminum, aluminum alloy, or the like; expandedmetal; or perforated metal can be used, for example. The metal foil canbe formed to a thickness of about greater than or equal to 15 μm andless than or equal to 25 μm, for example. The positive-electrode mixturelayer 111 b includes the aforementioned positive-electrode material. Inaddition, the positive-electrode mixture layer 111 b may also include aconductive material, binder, and the like.

The negative electrode 112 includes a negative-electrode currentcollector 112 a and a negative-electrode mixture layer 112 b formed onthe surface of the negative-electrode current collector 112 a. For thenegative-electrode current collector 112 a, a metal foil of copper,copper alloy, nickel, nickel alloy, or the like; expanded metal;perforated metal; or the like can be used. The metal foil can be formedto a thickness of about greater than or equal to 7 μm and less than orequal to 10 μm, for example. The negative-electrode mixture layer 112 bincludes a negative-electrode active material that is used for commonlithium ion secondary batteries. In addition, the negative-electrodemixture layer 112 b may also include a conductive material, binder, andthe like.

For the negative-electrode active material, for example, one or more ofa carbon material, a metallic material, and metal oxide can be used. Forthe carbon material, graphites such as natural graphite or artificialgraphite; carbides such as coke or pitch; amorphous carbon; carbonfibers; or the like can be used. For the metallic material, lithium,silicon, tin, aluminum, indium, gallium, magnesium, or alloys thereofcan be used. For the metal oxide material, metal oxide including tin,silicon, lithium, titanium, or the like can be used.

For the separator 113, a microporous film or a nonwoven fabric ofpolyolefin-based resin, such as polyethylene, polypropylene,polyethylene-polypropylene copolymer; polyamide resin; or aramid resincan be used, for example.

The positive electrode 111 and the negative electrode 112 can be eachformed through a mixture preparing step, a mixture coating step, and amolding step, for example. In the mixture preparing step, a mixtureslurry is prepared by stirring and homogenizing a positive-electrodeactive material or a negative-electrode active material together with asolution containing a conductive material and a binder, for example,using a stirring means such as a planetary mixer, a dispersion mixer, ora rotation-revolution mixer, for example.

For the conductive material, conductive materials used for commonlithium ion secondary batteries can be used. Specifically, carbonparticles or carbon fibers, such as a graphite powder, acetylene black,furnace black, thermal black, or channel black, can be used as theconductive material. The content of the conductive material can be setto about greater than or equal to 3 mass % and less than or equal to 10mass % relative to the total mass of the mixture, for example.

For the binder, binders used for common lithium ion secondary batteriescan be used. Specifically, polyvinylidene fluoride (PVDF),polytetrafluoroethylene, polyhexafluoropropylene, styrene-butadienerubber, carboxymethylcellulose, polyacrylonitrile, or modifiedpolyacrylonitrile can be used as the binder, for example. The content ofthe binder can be set to about greater than or equal to 2 mass % andless than or equal to 10 mass % relative to the total mass of themixture, for example. The negative-electrode active material and thebinder are desirably mixed at a weight ratio of 95:5, for example.

The solvent of the solution can be selected from N-methylpyrrolidone,water, N,N-dimethylformamide, N,N-dimethylacetamide, methanol, ethanol,propanol, isopropanol, ethylene glycol, diethylene glycol, glycerin,dimethyl sulfoxide, tetrahydrofuran, or the like in accordance with thetype of the binder used.

In the mixture coating step, first, the surfaces of thepositive-electrode current collector 111 a and the negative-electrodecurrent collector 112 a are coated with the mixture slurry containingthe positive-electrode active material and the mixture slurry containingthe negative-electrode active material, which have been prepared in themixture preparing step, respectively, using a coating means such as abar coater, a doctor blade, or a transfer roller, for example. Next,heat treatment is applied to the positive-electrode current collector111 a and the negative-electrode current collector 112 a each coatedwith the mixture slurry, whereby the solvent of the solution containedin the mixture slurry is volatilized or vaporized to be removed, andthus, the positive-electrode mixture layer 111 b and thenegative-electrode mixture layer 112 b are formed on the surfaces of thepositive-electrode current collector 111 a and the negative-electrodecurrent collector 112 a, respectively.

In the molding step, first, the positive-electrode mixture layer 111 bformed on the surface of the positive-electrode current collector 111 aand the negative-electrode mixture layer 112 b formed on the surface ofthe negative-electrode current collector 112 a are subjected to pressuremolding using a pressure means, such as a roll press, for example.Accordingly, the positive-electrode mixture layer 111 b can be pressedto a thickness of about greater than or equal to 100 μm and less than orequal to 300 μm, for example, while the negative-electrode mixture layer112 b can be pressed to a thickness of about greater than or equal to 20μm and less than or equal to 150 μm, for example. After that, thepositive-electrode current collector 111 a and the positive-electrodemixture layer 111 b as well as the negative-electrode current collector112 a and the negative-electrode mixture layer 112 b are each cut into along strip shape, whereby the positive electrode 111 and the negativeelectrode 112 can be produced.

The thus produced positive electrode 111 and negative electrode 112 arewound around the central axis of the winding in a state in which thepositive electrode 111 and the negative electrode 112 are opposite eachother with the separator 113 interposed therebetween, whereby the woundelectrode group 110 is formed. In the wound electrode group 110, thenegative-electrode current collector 112 a is connected to the bottom ofthe battery can 101 via the negative-electrode lead strip 104, while thepositive-electrode current collector 111 a is connected to the batterylid 102 via the positive-electrode lead strip 103, and the woundelectrode group 110 is housed in the battery can 101 while short betweenthe battery can 101 and the battery lid 102 is prevented by theinsulating plate 105 or the like. After that, a nonaqueous electrolyticsolution is injected into the battery can 101, and the battery lid 102is secured to the battery can 101 via the sealing material 106 so as tohermetically seal the battery can 101, whereby the lithium ion secondarybattery 100 can be produced.

For the electrolytic solution injected into the battery can 101, it ispreferable to use an electrolytic solution obtained by dissolving, as anelectrolyte, lithium hexafluorophosphate (LiPF₆), lithiumtetrafluoroborate (LiBF₄), lithium perchlorate (LiClO₄), or the like ina solvent such as diethyl carbonate (DEC), dimethyl carbonate (DMC),ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate(VC), methyl acetate (MA), ethyl methyl carbonate (EMC), or methylpropyl carbonate (MPC). The concentration of the electrolyte isdesirably set to greater than or equal to 0.7 M and less than or equalto 1.5 M. In addition, the electrolytic solution may also be mixed witha compound containing an anhydrous carboxylic acid group, a compoundcontaining a sulfur element, such as propanesultone, or a compoundcontaining boron. Such compounds are added in order to suppressreductive decomposition of the electrolytic solution on the surface ofthe negative electrode, prevent reduction deposition of a metal element,such as manganese eluted from the positive electrode, on the negativeelectrode, improve the ion conductivity of the electrolytic solution,and increase the flame resistance of the electrolytic solution, and canbe appropriately selected in accordance with purposes.

The lithium ion secondary battery 100 with the aforementioned structureis configured such that power supplied from the outside can be stored inthe wound electrode group 110 using the battery lid 102 as apositive-electrode outside terminal and using the bottom of the batterycan 101 as a negative-electrode outside terminal, and the power storedin the wound electrode group 110 can be supplied to the outside deviceand the like. In this manner, the lithium ion secondary battery 100 inthis embodiment can be used as a small power supply for portableelectronic devices or home electric appliances; an uninterruptible powersupply; a stationary power supply such as a power leveling device; or adrive power supply for vessels, railways, hybrid vehicles, electricvehicles, or the like.

In the lithium ion secondary battery 100 in this embodiment, the mixturelayer 111 b of the positive electrode 111 contains the aforementionedpositive-electrode material. Therefore, a secondary battery with ahigher charge-discharge capacity than those of the conventionalsecondary batteries, a suppressed resistance increase rate, excellentcycle characteristics, and excellent low-temperature outputcharacteristics is provided.

Although the embodiments of the present invention have been described indetail with reference to the drawings, specific structures are notlimited thereto, and any design changes and the like that may occurwithin the spirit and scope of the present invention are all included inthe present invention.

EXAMPLES

Hereinafter, Examples of the positive-electrode material and the methodfor producing the positive-electrode material of the present invention,and Comparative Examples that are not included in the present inventionwill be described.

Example 1

A positive-electrode material of Example 1 was produced in accordancewith the following procedures. First, lithium carbonate, nickelhydroxide, cobalt carbonate, manganese carbonate, and atitanium-containing chelating agent (titanium lactate ammonium salt)were prepared as the starting materials of the positive-electrodematerial. Next, a mixing step of mixing such starting materials wasperformed. Specifically, the starting materials were weighed so that theatomic ratio of Li:Ni:Co:Mn became 1.04:0.80:0.15:0.05.

The mixing step was performed by adding a titanium-containing chelatingagent to the aforementioned weighed starting materials so that thecontent of Ti became 1 mol provided that the total number of moles ofNi, Co, and Mn was 100 mol, and grinding the mixture with a grinder andwet-mixing it to prepare a slurry. Then, the obtained slurry was driedwith a spray dryer to obtain a mixed powder that is the mixture of theaforementioned starting materials. The sizes of the secondary particlesof the obtained mixed powder were about 10 μm.

Next, a firing step of firing the mixture obtained in the mixing stepwas performed. Specifically, the mixed powder obtained in the mixingstep was fired in a firing step including a first heat treatment step, asecond heat treatment step, and a third heat treatment step.

In the first heat treatment step, an alumina container with a depth of300 mm, a width of 300 mm, and a height of 100 mm was filled with a 1 kgmixture obtained in the mixing step, and heat treatment was performedwith a continuous transfer furnace at a heat treatment temperature of350° C. under an air atmosphere for 1 hour, whereby a powder of a firstprecursor was obtained. In this step, water vapor is generated alongwith the thermal decomposition of nickel hydroxide, and carbon dioxideis also generated along with the thermal decomposition of cobaltcarbonate and manganese carbonate.

In the second heat treatment step, the powder of the first precursorobtained in the first heat treatment step is subjected to heat treatmentat a heat treatment temperature of 600° C. in an oxygen stream for 10hours, using a continuous transfer furnace, which has an atmospheresubstituted so that it has greater than or equal to 90% oxygenconcentration inside the furnace, whereby a powder of a second precursorwas obtained. In this step, cobalt carbonate and manganese carbonatethat have not reacted completely in the first heat treatment step arethermally decomposed, thus generating carbon dioxide. Further, as thereaction of lithium carbonate with Ni, Co, Mn, and Ti progresses, carbondioxide is generated.

In the third heat treatment step, the powder of the second precursorobtained in the second heat treatment step is subjected to heattreatment at a heat treatment temperature of 785° C. in an oxygen streamfor 10 hours, using a continuous transfer furnace, which has anatmosphere substituted so that it has greater than or equal to 90%oxygen concentration inside the furnace, whereby a powder of a lithiumcomplex compound was obtained. The obtained powder of the lithiumcomplex compound was classified using a sieve with an opening of lessthan or equal to 53 μm, and a positive-electrode material was thusformed with the classified powder of the lithium complex compound.

The element ratio of the positive-electrode material of Example 1obtained through the aforementioned steps was measured with ICP toobtain a composition formula of a lithium complex compound that formsthe positive-electrode material of Example 1. In addition, thepositive-electrode material of Example 1 was analyzed through XPS tomeasure a Ti2p spectrum, which was then fitted using two spectra derivedfrom Ti₂O₃(Ti³⁺) and TiO₂(Ti⁴⁺) using analysis software “PHI MultiPak(registered trademark)” produced by ULVAC-PHI, Inc. At that time,fitting was performed so that “Chi-squared” (Pearson's chi-square),which is a reliability parameter for fitting, became less than or equalto 10. The obtained area ratio between Ti³⁺ and Ti⁴⁺ was set to theatomic ratio Ti³⁺/Ti⁴⁺ between Ti³⁺ and Ti⁴⁺ of the lithium complexcompound that forms the positive-electrode active material of Example 1.

In addition, in order to evaluate the stability of the oxide, the weightreduction rate of the positive-electrode material of Example 1 when thetemperature was increased from the room temperature to 1000° C. at arate of 10° C./minute in a nitrogen atmosphere was measured throughthermogravimetric analysis.

Further, positive-electrode material particles with secondary particlesizes of 5 to 10 μm were selected using an optical microscope, and eachparticle was compressed with a flat indenter with a diameter of 50 μm ata load rate of 0.47 m·N/s using a micro compression testing machine(MCT-510 produced by Shimadzu Scientific Instruments) so as to measurethe fracture strength of the particle. In addition, after thepositive-electrode material and the glassware were dried in a vacuum at120° C. for 2 hours, the specific surface area of the positive-electrodematerial was measured using an automatic specific surface area/pore sizedistribution measuring device (BELSORP-mini produced by MicrotracBELCorp.).

Example 2

A positive-electrode material of Example 2 was obtained by producing apositive-electrode material in the same manner as in Example 1 exceptthat the heat treatment temperature in the third heat treatment step wasset to 800° C. The obtained positive-electrode material of Example 2 wasanalyzed as with the positive-electrode material of Example 1 so as toobtain a composition formula, the atomic ratio Ti³⁺/Ti⁴⁺, the weightreduction rate (through thermogravimetric analysis), the particlefracture strength, and the specific surface area of a lithium complexcompound that forms the positive-electrode material of Example 2.

Example 3

A positive-electrode material of Example 3 was obtained by producing apositive-electrode material in the same manner as in Example 1 exceptthat the amount of the titanium-containing chelating agent added in themixing step was set so that the number of moles of Ti became 2. Theobtained positive-electrode material of Example 3 was analyzed as withthe positive-electrode material of Example 1 so as to obtain acomposition formula, the atomic ratio Ti³⁺/Ti⁴⁺, the particle fracturestrength, and the specific surface area of a lithium complex compoundthat forms the positive-electrode material of Example 3.

Example 4

A positive-electrode material of Example 4 was obtained by producing apositive-electrode material in the same manner as in Example 1 exceptthat the amount of the titanium-containing chelating agent added in themixing step was set so that the number of moles of Ti became 3. Theobtained positive-electrode material of Example 4 was analyzed as withthe positive-electrode material of Example 1 so as to obtain acomposition formula, the atomic ratio Ti³⁺/Ti⁴⁺, the particle fracturestrength, and the specific surface area of a lithium complex compoundthat forms the positive-electrode material of Example 4.

Example 5

A positive-electrode material of Example 5 was obtained by producing apositive-electrode material in the same manner as in Example 2 exceptthat the Li content was increased such that the atomic ratio ofLi:Ni:Co:Mn became 1.08:0.80:0.15:0.05 in the mixing step. The obtainedpositive-electrode material of Example 5 was analyzed as with thepositive-electrode material of Example 1 so as to obtain a compositionformula, the atomic ratio Ti³⁺/Ti⁴⁺, the particle fracture strength, andthe specific surface area of a lithium complex compound that forms thepositive-electrode material of Example 5.

Example 6

A positive-electrode material of Example 6 was obtained by producing apositive-electrode material in the same manner as in Example 2 exceptthat the Li content was reduced such that the atomic ratio ofLi:Ni:Co:Mn became 1.02:0.80:0.15:0.05 in the mixing step. The obtainedpositive-electrode material of Example 6 was analyzed as with thepositive-electrode material of Example 1 so as to obtain a compositionformula, the atomic ratio Ti³⁺/Ti⁴⁺, the particle fracture strength, andthe specific surface area of a lithium complex compound that forms thepositive-electrode material of Example 6.

Example 7

Titanium oxide (TiO₂) was prepared as a titanium raw material, and Li,Ni, Co, Mn, and Ti were weighed so that the atomic ratio ofLi:Ni:Co:Mn:Ti became 1.04:0.79:0.15:0.05:0.01. Then, a mixing step wasperformed by grinding such materials with a grinder and wet-mixing it toprepare a slurry. In the mixing step, the materials were ground untilthe average particle size of the solid portion of the slurry became 0.15μm. Then, the obtained slurry was dried with a spray dryer to obtain amixed powder that is the mixture of the aforementioned startingmaterials. After that, a firing step was performed as in Example 2 toobtain a positive-electrode material of Example 7. The obtainedpositive-electrode material of Example 7 was analyzed as with thepositive-electrode material of Example 1 so as to obtain a compositionformula, the atomic ratio Ti³⁺/Ti⁴⁺, the particle fracture strength, andthe specific surface area of a lithium complex compound that forms thepositive-electrode material of Example 7.

Example 8

A positive-electrode material of Example 8 was obtained by producing apositive-electrode material in the same manner as in Example 7 exceptthat the average particle size of the solid portion of the slurry became0.35 μm in the mixing step. The obtained positive-electrode material ofExample 8 was analyzed as with the positive-electrode material ofExample 1 so as to obtain a composition formula, the atomic ratioTi³⁺/Ti⁴⁺, the particle fracture strength, and the specific surface areaof a lithium complex compound that forms the positive-electrode materialof Example 8.

Example 9

A positive-electrode material of Example 9 was obtained by producing apositive-electrode material in the same manner as in Example 1 exceptthat the heat treatment temperature in the third heat treatment step wasset to 815° C. The obtained positive-electrode material of Example 9 wasanalyzed as with the positive-electrode material of Example 1 so as toobtain a composition formula, the atomic ratio Ti³⁺/Ti⁴⁺, the particlefracture strength, and the specific surface area of a lithium complexcompound that forms the positive-electrode material of Example 9.

Comparative Example 1

A positive-electrode material of Comparative Example 1 was obtained byproducing a positive-electrode material in the same manner as in Example1 except that titanium oxide was not added and the heat treatmenttemperature in the third heat treatment step was set to 770° C. Theobtained positive-electrode material of Comparative Example 1 wasanalyzed as with the positive-electrode material of Example 1 so as toobtain a composition formula, the weight reduction rate (throughthermogravimetric analysis), the particle fracture strength, and thespecific surface area of a lithium complex compound that forms thepositive-electrode material of Comparative Example 1.

Comparative Example 2

A positive-electrode material of Comparative Example 2 was obtained byproducing a positive-electrode material in the same manner as in Example1 except that the heat treatment temperature in the third heat treatmentstep was set to 755° C. The obtained positive-electrode material ofComparative Example 2 was analyzed as with the positive-electrodematerial of Example 1 so as to obtain a composition formula, the atomicratio Ti³⁺/Ti⁴⁺, the weight reduction rate (through thermogravimetricanalysis), the particle fracture strength, and the specific surface areaof a lithium complex compound that forms the positive-electrode materialof Comparative Example 2.

Comparative Example 3

A positive-electrode material of Comparative Example 3 was obtained byproducing a positive-electrode material in the same manner as in Example1 except that the heat treatment temperature in the third heat treatmentstep was set to 770° C. The obtained positive-electrode material ofComparative Example 3 was analyzed as with the positive-electrodematerial of Example 1 so as to obtain a composition formula, the atomicratio Ti³⁺/Ti⁴⁺, the weight reduction rate (through thermogravimetricanalysis), the particle fracture strength, and the specific surface areaof a lithium complex compound that forms the positive-electrode materialof Comparative Example 3.

Comparative Example 4

A positive-electrode material of Comparative Example 4 was obtained byproducing a positive-electrode material in the same manner as in Example1 except that the amount of the titanium-containing chelating agentadded in the mixing step was set so that the number of moles of Tibecame 4. The obtained positive-electrode material of ComparativeExample 4 was analyzed as with the positive-electrode material ofExample 1 so as to obtain a composition formula, the atomic ratioTi³⁺/Ti⁴⁺, the particle fracture strength, and the specific surface areaof a lithium complex compound that forms the positive-electrode materialof Comparative Example 4.

Table 1A below shows the composition formula, the value of a/e in thecomposition formula, the heat treatment temperature (firing temperature)in the third heat treatment step included in the firing step, the typeof the Ti raw material, and the particle size after grinding of each ofthe positive-electrode materials of Example 1 to Comparative Example 4.In addition, Table 1A also shows the atomic ratio Ti³⁺/Ti⁴⁺ between Ti³⁺and Ti⁴⁺ of each of the lithium complex compounds that form thepositive-electrode materials of Examples 1 to 9 and Comparative Examples2 to 4.

TABLE 1A Composition Formula Li_((1+a))Ni_(b)Mn_(c)Co_(d)Ti_(e)O_(2+α)a/e Baking Temperature (° C.) Type of Ti Raw Material Particle Sizeafter Grinding $\frac{{Ti}^{3 +}}{{Ti}^{4 +}}$ Example 1Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 785 Chelate 1.9Example 2 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 800Chelate 2.8 Example 3Li_(1.02)Ni_(0.78)Mn_(0.05)Co_(0.15)Ti_(0.02)O_(2+α) 1 785 Chelate 2.0Example 4 Li_(1.02)Ni_(0.78)Mn_(0.04)Co_(0.16)Ti_(0.03)O_(2+α) 0.66 785Chelate 2.1 Example 5Li_(1.06)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 6 800 Chelate 1.5Example 6 Li_(1.00)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 0 800Chelate 2.9 Example 7Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 800 TiO₂ 0.15 μm2.7 Example 8 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 800TiO₂ 0.35 μm 1.8 Example 9Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 815 Chelate 2.9Comparative Li_(1.02)Ni_(0.80)Mn_(0.05)Co_(0.15)O_(2+α) — 770 — —Example 1 ComparativeLi_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 755 Chelate 1.3Example 2 ComparativeLi_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 770 Chelate 1.4Example 3 ComparativeLi_(1.02)Ni_(0.77)Mn_(0.04)Co_(0.15)Ti_(0.04)O_(2+α) 0.5 785 Chelate 1.3Example 4

Table 1B below shows the composition formula of each of thepositive-electrode materials of Example 1 to Comparative Example 4, andthe weight reduction rate of each of the positive-electrode materials ofExamples 1 and 2 and Comparative Examples 1 to 3 determined throughthermogravimetric analysis when the temperature was increased from theroom temperature to 1000° C. at a rate of 10° C./minute. In addition,Table 1B below also shows the particle fracture strength and thespecific surface area of each of the positive-electrode materials ofExample 1 to Comparative Example 4.

TABLE 1B Weight Particle Specific Composition Formula Reduction RateFracture Surface Area Li_((1+a))Ni_(b)Mn_(c)Co_(d)Ti_(e)O_(2+α) @1000°C. (%) Strength (MPa) (m²/g) Example 1Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) −5.1 51 1.30Example 2 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) −5.9 670.76 Example 3 Li_(1.02)Ni_(0.78)Mn_(0.05)Co_(0.15)Ti_(0.02)O_(2+α) — 551.25 Example 4 Li_(1.02)Ni_(0.78)Mn_(0.04)Co_(0.15)Ti_(0.03)O_(2+α) — 531.33 Example 5 Li_(1.06)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) — 451.29 Example 6 Li_(1.00)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) — 740.81 Example 7 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) —138 0.62 Example 8 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α)— 83 0.85 Example 9 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α)— 153 0.47 Comparative Example 1Li_(1.02)Ni_(0.80)Mn_(0.05)Co_(0.15)O_(2+α) −5.8 109 0.86 ComparativeExample 2 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) −5.9 402.54 Comparative Example 3Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) −5.9 45 2.36Comparative Example 4Li_(1.02)Ni_(0.77)Mn_(0.04)Co_(0.15)Ti_(0.04)O_(2+α) — 42 1.81

The atomic ratio Ti³⁺/Ti⁴⁺ of the lithium composite oxide that forms thepositive-electrode active material of each of Examples 1 to 9 is greaterthan or equal to 1.5, more specifically, greater than or equal to 1.9except Example 5, whereas the atomic ratio Ti³⁺/Ti⁴⁺ of the lithiumcomposite oxide that forms the positive-electrode active material ofeach of Comparative Examples 2 to 4 is as low as less than 1.5.

In addition, Comparative Example 1 without Ti added thereto has aparticle fracture strength as high as greater than or equal to 100 MPa,while the other materials with Ti added thereto tend to have a lowparticle fracture strength. It is acceptable as long as the particlefracture strength is greater than or equal to about 45 MPa. Of thematerials with Ti added thereto, Examples 2, 6, and 7 each having ahigher atomic ratio Ti³⁺/Ti⁴⁺ of the lithium composite oxide have arelatively high particle fracture strength. In particular, Example 7 inwhich Ti oxide (TiO₂) was used as the Ti raw material has a highparticle fracture strength. In addition, Example 9 in which the firingtemperature was high has a specific surface area as small as less thanor equal to 0.5 m²/g, while each of Comparative Examples 2 and 3 inwhich the firing temperature was low has a specific surface area aslarge as greater than 2.0 m²/g.

FIG. 4A is a mapped image of a Ti element in a primary particle of thepositive-electrode material of Example 4. The mapped image of the Tielement shown in FIG. 4A was obtained by embedding thepositive-electrode material of Example 4 in resin, and then forming thematerial into a thin piece using a focused ion beam (FIB) and measuringthe material with an energy dispersive X-ray spectroscopy (TEM-EDX)using a transmission electron microscope. FIG. 4B is a graphillustrating the distance from the surface of the primary particle ofthe positive-electrode material shown in FIG. 4A and the compositionratio. The composition ratio shown in FIG. 4B is the composition ratioof each of Ni, Co, Mn, and Ti in a linear composition ratio analysisregion A-A shown in FIG. 4A. FIG. 5 is a graph showing the XRD spectrumof the positive-electrode material of Example 4.

As shown in FIGS. 4A and 4B, Ti is not substituted in the entire primaryparticle of the positive-electrode material, but is concentrated in aregion of about 5 nm from the surface of the primary particle. Thisshows that an agglomerated Ti layer having Ti³⁺ concentrated therein isformed on the surface of each secondary particle having agglomeratedprimary particles of the lithium complex compound.

In addition, as shown in FIG. 5, a covering layer (different phase) likeLiTiO₂ was not seen. The positive-electrode material for the lithium ionsecondary battery of Example 4 has a, which indicates the amount ofexcess or deficiency of Li in the composition formula, in the range of 0to 0.06, has e, which indicates the Ti content, in the range of 0.005 to0.15, and has a ratio a/e of less than or equal to 5. Accordingly, it isconsidered that a different phase like a Li—Ti—O compound was notgenerated.

Next, the lithium ion secondary batteries of Example 1 to ComparativeExample 4 were produced in accordance with the following proceduresusing the positive-electrode materials of Example 1 to ComparativeExample 4. First, a positive-electrode material, a binder, and aconductive material were mixed to prepare a positive-electrode mixtureslurry. Then, an aluminum foil with a thickness of 20 which is apositive-electrode current collector, was coated with the preparedpositive-electrode mixture slurry. Then, the slurry was dried at 120° C.and compression-molded with a press such that the electrode densitybecame 2.7 g/cm³. Then, the molded object was stamped into a disk with adiameter of 15 mm to produce a positive electrode. Then, a negativeelectrode was produced by using metallic lithium as a negative-electrodematerial.

Next, a lithium ion secondary battery was produced using the thusproduced positive electrode and negative electrode as well as anonaqueous electrolytic solution. For the nonaqueous electrolyticsolution, a solution obtained by dissolving LiPF₆ in a solvent, whichhas been obtained by mixing ethylene carbonate and dimethyl carbonate toattain a volume ratio of 3:7, so that the final concentration became 1.0mol/L was used.

Next, a charge-discharge test was conducted on each of the producedlithium ion secondary batteries of Example 1 to Comparative Example 4 tomeasure the initial discharge capacity. Charging was conducted with acharging current of 0.2 CA, and with a constant current and a constantvoltage up to a charging termination voltage of 4.3 V, while dischargingwas conducted with a discharging current of 0.2 CA, and with a constantcurrent up to a discharging termination voltage of 2.5 V. After that, 50cycles of charging and discharging were repeatedly conducted with acharging and discharging current of 1.0 CA, a charging terminationvoltage of 4.4 V, and a discharging termination voltage of 2.5 V. Thepercentage of the discharge capacity measured in the 50-th cycle dividedby the discharge capacity measured in the 1st cycle was defined as thecapacity retention rate.

Further, regarding Examples 2 and 5 to 8 and Comparative Examples 1 and3, the resistance increase rate as well as the resistance at −20° C. anda state-of-charge of 10% (10% SOC) was evaluated. For evaluating suchvalues, graphite was used as the negative-electrode material, andgraphite, methyl-cellulose sodium, and styrene/butadiene rubber weremixed at a mass ratio of 98:1:1. Then, a current collector of a copperfoil with a thickness of 10 μm was coated with the uniformly mixedslurry. After that, the slurry applied to the current collector wasdried at 120° C. and compression-molded with a press such that theelectrode density became 1.5 g/cm³.

With the negative electrode produced as above, a battery was produced insame manner as that described above. The thus produced battery wascharged with a constant current of 0.2 CA, a constant voltage of 4.2 V,and a cut current of 0.05 CA, and was then discharged with a constantcurrent of 0.2 CA and a termination voltage of 2.5 V, so that theobtained discharge capacity was determined as the rated voltage. Afterthat, the battery was charged again with a constant current of 0.2 CA, aconstant voltage of 4.2 V, and a cut current of 0.05 CA, so that avoltage at which a 10% charging capacity of the rated capacity wasattained was determined as the 10% SOC voltage.

Similarly, 20%, 50%, and 95% SOC voltages of the produced battery weredetermined. Further, after the produced battery was charged at 50° C. upto 95% SOC, a cycle of discharging the battery by 75% of the ratedcapacity was repeated 300 times, and the resistance of the battery afterit was discharged for 10 seconds at 50% SOC before and after the cycleswas measured so that the resistance increase rate before and after thecycles was evaluated. Similarly, after 10% SOC of the battery wasattained at 25° C., the resistance of the battery at −20° C. wasevaluated.

Table 2A below shows the composition formula, the particle fracturestrength, and the specific surface area of each of thepositive-electrode materials of Example 1 to Comparative Example 4. Inaddition, Table 2A also shows the atomic ratio Ti³⁺/Ti⁴⁺ between Ti³⁺and Ti⁴⁺ of the lithium complex compound that forms thepositive-electrode material of each of Examples 1 to 9 and ComparativeExamples 2 to 4 and the value a/e of the composition formula of thepositive-electrode material.

TABLE 2A Particle Specific Composition FormulaLi_((1+a))Ni_(b)Mn_(c)Co_(d)Ti_(e)O_(2+α) a/e$\frac{{Ti}^{3 +}}{{Ti}^{4 +}}$ Fracture Strength (MPa) Surface Area(m²/g) Example 1 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 21.9 51 1.30 Example 2Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 2.8 67 0.76Example 3 Li_(1.02)Ni_(0.78)Mn_(0.05)Co_(0.15)Ti_(0.02)O_(2+α) 1 2.0 551.25 Example 4 Li_(1.02)Ni_(0.78)Mn_(0.04)Co_(0.16)Ti_(0.03)O_(2+α) 0.662.1 53 1.33 Example 5Li_(1.06)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 6 1.5 45 1.29Example 6 Li_(1.00)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 0 2.9 740.81 Example 7 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 22.7 138 0.62 Example 8Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 1.8 83 0.85Example 9 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 2.9 1530.47 Comparative Li_(1.02)Ni_(0.80)Mn_(0.05)Co_(0.15)O_(2+α) — — 1090.86 Example 1 ComparativeLi_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 1.3 40 2.54Example 2 ComparativeLi_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 2 1.4 45 2.36Example 3 ComparativeLi_(1.02)Ni_(0.77)Mn_(0.04)Co_(0.15)Ti_(0.04)O_(2+α) 0.5 1.3 42 1.81Example 4

Table 2B below shows the 0.2 C initial discharge capacity and thecapacity retention rate after 50 cycles of each of the lithium ionsecondary batteries of Examples 1 to 9 and Comparative Examples 1 to 4.In addition, Table 2B also shows the resistance increase rate and the10% SOC resistance ratio at −20° C. of each of the secondary batteriesof Examples 2 and 5 to 8 and Comparative Examples 1 and 3.

TABLE 2B 0.2 C Capacity 10% SOC Discharge Retention ResistanceResistance Composition Formula Capacity Rate 50 Increase RatioLi_((1+a))Ni_(b)Mn_(c)Co_(d)Ti_(e)O_(2+α) (Ah/kg) Cycles (%) Rate (%) at−20° C. Example 1 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α)210 90.9 — — Example 2Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 190 92.9 53 0.73Example 3 Li_(1.02)Ni_(0.78)Mn_(0.05)Co_(0.15)Ti_(0.02)O_(2+α) 192 91.8— — Example 4 Li_(1.02)Ni_(0.78)Mn_(0.04)Co_(0.15)Ti_(0.03)O_(2+α) 19392.1 — — Example 5 Li_(1.06)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α)195 90.0 85 0.85 Example 6Li_(1.00)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 188 92.0 68 0.75Example 7 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 189 93.126 0.66 Example 8 Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α)183 90.2 63 0.80 Example 9Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 183 92.0 — —Comparative Example 1 Li_(1.02)Ni_(0.80)Mn_(0.05)Co_(0.15)O_(2+α) 19389.8 128  1   Comparative Example 2Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 183 86.5 — —Comparative Example 3Li_(1.02)Ni_(0.79)Mn_(0.05)Co_(0.15)Ti_(0.01)O_(2+α) 178 87.4 156  0.92Comparative Example 4Li_(1.02)Ni_(0.77)Mn_(0.04)Co_(0.15)Ti_(0.04)O_(2+α) 175 88.3 — —

It is found that the capacity retention rate of each of the lithium ionsecondary batteries of Examples 1 to 9 with Ti added thereto is over90%, which is higher than the capacity retention rate of the lithium ionsecondary battery of Comparative Example 1 without Ti added thereto.That is, with the addition of Ti, it is possible to suppress generationof a different phase in the grain boundaries of the particles of thepositive-electrode material and thus improve the particle fracturestrength as described below, and also suppress generation of Ni oxide(NiO-like different phase) on the surfaces of the particles and thussuppress the resistance increase rate. Consequently, it was confirmedthat the cycle characteristics of the positive-electrode material wereimproved. Meanwhile, it was also confirmed that since each of thelithium ion secondary batteries of Comparative Examples 2 to 4 has a lowcapacity retention rate and a low 0.2 C initial discharge capacity, onlythe addition of Ti is not sufficient, but the battery performance can beeffectively improved by performing firing at an appropriate temperatureand setting the atomic ratio Ti³⁺/Ti⁴⁺ to greater than or equal to 1.5.

FIG. 6 is a graph showing the particle fracture strength of each of thepositive-electrode materials of Examples 2, 5, and 6 and ComparativeExample 1. The particle fracture strength of each of Examples 2, 5 and 6is lower than that of Comparative Example 1 without Ti added thereto.That is, when Ti is added, the particle fracture strength of thepositive-electrode material becomes lower than that when Ti is notadded.

However, regarding Example 2 having a higher atomic ratio Ti³⁺/Ti⁴⁺ thanthat of Example 5, the particle fracture strength of thepositive-electrode material is improved than that of Example 5. Further,regarding Example 6 having a higher atomic ratio Ti³⁺/Ti⁴⁺ than that ofExample 2, the particle fracture strength of the positive-electrodematerial is further improved than that of Example 2. That is, it wasconfirmed that as long as the atomic ratio Ti³⁺/Ti⁴⁺ is in the range of1.5 to 20, a sufficient particle fracture strength can be maintainedeven if Ti is added.

It was also confirmed that regarding Example 2 having high Ti³⁺/Ti⁴⁺ anda high particle fracture strength, the resistance increase rate issignificantly reduced than that of Comparative Example 1 without Tiadded thereto, and regarding the resistance increase rate also, theeffect of improving the cycle characteristics is obtained.

FIG. 7 is a graph showing the relationship between the resistanceincrease rate of each of the secondary batteries of Examples 2 and 5 to8 and Comparative Examples 1 and 3 and the specific surface area of eachpositive-electrode material. Regarding the secondary batteries otherthan the secondary battery of Comparative Example 1 in which apositive-electrode material without Ti added thereto was used, it wasconfirmed that the resistance increase rate can be effectivelysuppressed as long as the BET specific surface area of thepositive-electrode material is in the range of 0.2 m²/g to 2.0 m²/g.

In addition, regarding Example 9 having a specific surface area as smallas 0.47 m²/g, the 0.2 C discharge capacity is 183 Ah/Kg, which is lowerthan those of the other Examples. Thus, it is found that in order toobtain a high capacity, the specific surface area is desirably greaterthan or equal to 0.5 m²/g.

Regarding Example 5 having a high Li composition ratio, Ti³⁺/Ti⁴⁺ islower than that of Example 2, and the particle strength is lower and theresistance increase rate is higher. This is considered to be due to thereason that generation of a different phase like Li₂TiO₃ has beenpromoted.

In addition, regarding Example 7 in which TiO₂ was used as the Ti rawmaterial and the particle strength is high, the resistance increase rateis further lower than that of Example 5. However, regarding Example 8 inwhich TiO₂ was similarly used as the Ti raw material, grinding was notperformed sufficiently. Therefore, Ti³⁺/Ti⁴⁺ is low and the resistanceincrease rate is higher than that of Example 7.

Further, at a low temperature where the proportion of the resistance ofthe positive electrode is high, specifically, at −20° C. that is a lowSOC condition, the 10% SOC resistance of each of Examples 2 and 5 to 8,in particular, is significantly lower than those of Comparative Examples1 and 3. It was confirmed that regarding a positive-electrode materialwith high Ti³⁺/Ti⁴⁺, reactions at the positive electrode are notinhibited, and rather, the resistance becomes low at low SOC.

FIG. 8A is a microscope photograph of a cross-section of an area aroundthe surface of a particle of the positive-electrode material for thelithium ion secondary battery of Example 2 after 300 cycles. Thepositive-electrode material for the lithium ion secondary battery ofExample 2 has a, which indicates the amount of excess or deficiency ofLi in the composition formula, in the range of 0 to 0.06, and has e,which indicates the Ti content, in the range of 0.005 to 0.15. When Tiis contained in a lithium complex compound that is a positive-electrodematerial, the thickness of a layer of a NiO-like different phase issuppressed to about 2 nm. It should be noted that the outermost surfaceof each particle of the positive-electrode material has formed thereon alayer of a re-deposited substance and a surface film.

FIG. 8B is a microscope photograph of a cross-section of an area aroundthe surface a particle of the positive-electrode material for thelithium ion secondary battery of Comparative Example 1 after 300 cycles.The positive-electrode material for the lithium ion secondary battery ofComparative Example 1 has a, which indicates the amount of excess ordeficiency of Li in the composition formula, in the range of 0 to 0.06,but does not contain Ti. Therefore, a layer of a NiO-like differentphase is formed to a thickness of about 6 nm, which means that adifferent phase is more likely to be generated than when Ti is added. Itshould be noted that the outermost surface of each particle of thepositive-electrode material also has formed thereon a layer of are-deposited substance and a surface film.

Regarding the positive-electrode materials used for the secondarybatteries of Example 2 and Comparative Example 1, the film thickness ofthe NiO-like different phase generated before or after the cycles wasevaluated using electron energy loss spectroscopy (TEM-EELS).

FIG. 9A is a microscope photograph of a cross-section of thepositive-electrode material for the secondary battery of Example 2 after0 cycle. FIGS. 9B and 9C show the results of measuring Ni and O throughelectron energy loss spectroscopy (TEM-EELS) at each distance from thesurface of the positive-electrode material shown in FIG. 9A (first tosixth distances; seventh distance indicates NiO as a reference sample).

Meanwhile, FIG. 10A is a microscope photograph of a cross-section of thepositive-electrode material for the secondary battery of ComparativeExample 1 after 0 cycle. FIGS. 10B and 10C show the measurement resultsof TEM-EELS at each distance from the surface of the positive-electrodematerial shown in FIG. 10A (first to sixth distances; seventh distanceindicates NiO on the surface).

FIG. 11A is a microscope photograph of a cross-section of thepositive-electrode material for the secondary battery of Example 2 after300 cycles. FIGS. 11B and 11C show the results of measuring an areaaround the surface of the positive-electrode material along A-A shown inFIG. 11A through TEM-EELS.

Meanwhile, FIG. 12A is a microscope photograph of a cross-section of thepositive-electrode material for the secondary battery of ComparativeExample 1 after 300 cycles. FIGS. 12B and 12C show the results ofmeasuring an area around the surface of the positive-electrode materialalong A-A shown in FIG. 12A through TEM-EELS.

A peak derived from Ni²⁺ was observed in the range of about 3 nm fromthe surface of the positive-electrode material for the secondary batteryof Example 2 after 0 cycle (FIG. 9B). The spectrum of 0 (FIG. 9C) alsoindicates that at a distance of up to 3 nm from the surface, a spectralshape differs from that of the inside of the particle, and a differentphase thus appears in the range of 3 nm from the surface layer. From theresults, it is found that the positive-electrode material for thesecondary battery of Example 2 after 0 cycle has a NiO-like differentphase generated in the range of about 3 nm from the surface. Regardingthe positive-electrode material used for the secondary battery ofExample 2, a peak derived from Ni²⁺ was observed in the range of about 3nm from the surface even after 300 cycles (FIGS. 11B and 11C). From theresults, it is found that the positive-electrode material used for thesecondary battery of Example 2 undergoes no change in the thickness ofthe NiO-like different phase before and after the cycles.

Meanwhile, regarding the positive-electrode material for the secondarybattery of Comparative Example 1 after 0 cycle, a peak derived from Ni²⁺was observed in the range of about 1 nm from the surface (FIGS. 10B and10C). After 300 cycles, a peak derived from Ni²⁺ was observed in therange of about 6 nm from the surface (FIGS. 12B and 12C). From theresults, it is found that the positive-electrode material used for thesecondary battery of Comparative Example 1 has an increased thickness ofthe NiO-like different phase after 300 cycles.

The aforementioned results can confirm that adding Ti to a lithiumcomplex compound can suppress generation of a NiO-like different phaseafter the cycles. Accordingly, it is estimated in Example 2 that theresistance increase rate is suppressed and the cycle characteristics areimproved. It was also confirmed that the same holds true for the otherExamples.

Example 10

The positive-electrode material of Example 2 was subjected to surfacetreatment in accordance with the following procedures. First, lithiumhexafluorophosphate (LiPF₆) and borate ester represented bytriisopropoxyboroxin ((BO)₃(O(CH)(CH₃)₂)₃) were dissolved in dimethylcarbonate (DMC). Then, a positive-electrode active material was put intoand immersed in such organic solvent, which was then stirred for 2hours. At this time, the amount of borate ester was adjusted such thatit became 1 mass % relative to the positive-electrode active material.After that, a powder obtained by suction-filtrating DMC was washed withDMC three times. The washed powder was then dried in a vacuum at 120° C.for 1 hour, whereby the positive-electrode material of Example 10 wasobtained.

The surfaces of the positive-electrode material of Example 2 and thepositive-electrode material of Example 10 shown in Tables 2A and 2B wereanalyzed through X-ray photoelectron spectroscopy (XPS). Regarding abinding spectrum of a Ni-2p2/3 that is the main component of thetransition metal, Table 3 shows the results of performing fittinganalysis of three components including a spectrum with a binding energyof 855.7±0.5 eV derived from Ni—O, a spectrum with a binding energy of857.4±0.5 eV derived from Ni—F, and a spectrum with a binding energy of862±0.5 eV that is the average of the satellite peaks of the twocomponents. Table 3 also shows the area ratio of each spectrum to thetotal sum of Ni—O and Ni—F. It should be noted that the satellite peakswere not taken into consideration for the area ratio of the total sum ofNi because they are “satellite.”

TABLE 3 Binding Energy Area Ratio Component (eV) FWHM (%) Example 2 Ni—O855.9 2.707 86.3 Ni—F 857.9 2.432 13.7 Satellite 861.9 3.880 — Example10 Ni—O 855.5 2.658 76.0 Ni—F 857.1 3.160 24.0 Satellite 861.6 3.758 —

The area ratio of Ni—F of Example 2 is 13.7%, while the area ratio ofNi—F of Example 10 is 24.0%. That is, the area ratio of Ni—F of Example10 is increased than that of Example 2 that has not been subjected tosurface treatment. The results can confirm that the surface of thepositive-electrode material used in Example 10 is fluorinated.

Next, the lithium ion secondary battery of Example 10 was produced inthe same manner as the lithium ion secondary battery of Example 1 usingthe positive-electrode material of Example 10, and then, the 0.2Cdischarge capacity, the capacity retention rate, the resistance increaserate, and the 10% SOC resistance ratio were measured. Table 4 belowshows the measurement results of the lithium secondary battery ofExample 10 together with the measurement results of the lithium ionsecondary battery of Example 2.

TABLE 4 0.2 C Capacity 10% SOC Discharge Retention Rate ResistanceResistance Capacity 50 Cycles Increase Ratio (Ah/kg) (%) Rate (%) at−20° C. Example 2 190 92.9 53 0.73 Example 10 188 93.3 27 0.64

The resistance increase rate of the secondary battery of Example 2 is53%, while the resistance increase rate of the secondary battery ofExample 10 is 27%. That is, it was confirmed that applying surfacetreatment to the positive-electrode material by immersing it in anorganic solvent containing dissolved therein a boroxine compoundrepresented by Formula (BO)₃(OR)₃ and fluoride can fluorinate thesurfaces of the secondary particles and can further suppress theresistance increase rate.

REFERENCE SIGNS LIST

-   100 Lithium ion secondary battery-   S1 Mixing step-   S2 Firing step-   S3 Immersing step

The invention claimed is:
 1. A method for producing a positive-electrodematerial for a lithium ion secondary battery, comprising: a mixing stepof mixing a lithium-containing compound with compounds each containing ametal element other than Li in the following Formula (1), therebyobtaining a mixture; and firing the mixture under an oxidizingatmosphere to obtain a lithium complex compound, the lithium complexcompound being represented by the following Formula (1) and having anatomic ratio Ti³⁺/Ti⁴⁺ between Ti³⁺ and Ti⁴⁺, as determined throughX-ray photoelectron spectroscopy, of greater than or equal to 1.5 andless than or equal to 20, wherein the compounds each containing a metalother than Li in the mixing step comprise an organic titanium compoundas a Ti-containing compound:Li_(1+a)Ni_(b)Mn_(c)Co_(d)Ti_(e)M_(f)O_(2+α)  (1), where in the Formula(1), M is at least one element selected from the group consisting of Mg,Al, Zr, Mo, and Nb, and a, b, c, d, e, f, and α are numbers satisfying−0.1≤a≤0.2, 0.7<b≤0.9, 0≤c<0.3, 0≤d<0.3, 0<e≤0.25, 0≤f<0.3, b+c+d+e+f=1,and −0.2≤α≤0.2.
 2. The method for producing a positive-electrodematerial for a lithium ion secondary battery according to claim 1,wherein the organic titanium compound is a titanium-containing chelatingagent.
 3. A method for producing a positive-electrode material for alithium ion secondary battery, comprising: a mixing step of mixing alithium-containing compound with compounds each containing a metalelement other than Li in the following Formula (1), thereby obtaining amixture; and firing the mixture under an oxidizing atmosphere to obtaina lithium complex compound, the lithium complex compound beingrepresented by the following Formula (1) and having an atomic ratioTi³⁺/Ti⁴⁺ between Ti³⁺ and Ti⁴⁺, as determined through X-rayphotoelectron spectroscopy, of greater than or equal to 1.5 and lessthan or equal to 20, wherein the compounds each containing a metal otherthan Li in the mixing step comprise titanium oxide as a Ti-containingcompound:Li_(1+a)Ni_(b)Mn_(c)Co_(d)Ti_(e)M_(f)O_(2+α)  (1), where in the Formula(1), M is at least one element selected from the group consisting of Mg,Al, Zr, Mo, and Nb, and a, b, c, d, e, f, and α are numbers satisfying−0.1≤a≤0.2, 0.7<b≤0.9, 0≤c<0.3, 0≤d<0.3, 0<e≤0.25, 0≤f<0.3, b+c+d+e+f=1,and −0.2≤α≤0.2.